The invention relates to a detection arrangement for observing infrared radiation emitting or reflecting objects, comprising a two-dimensional array of Nxc3x97M detector elements D(i,j), ixe2x89xa6N, jxe2x89xa6M, arranged in rows and columns; imaging means, for imaging objects and their environment on the array; image processing means, connected to the array, for periodically generating an image-representing two-dimensional matrix of numbers S(i,j), ixe2x89xa6N, jxe2x89xa6M, and compensation means, for compensating an offset for individual detector elements.
A detection arrangement of this type is known in the art, including compensation means for compensating the offset. There even exists a multiplicity of solutions, each however carrying specific drawbacks.
From EP-A- 0.601.534 compensation means are known in the form of a memory, in which for each detector element the response is set down as a function of incident radiation. The disadvantage of these known compensation means is that insufficient account is taken of the effect of ageing of the detector elements and of the fact that also the temperature of the housing of the detector elements comes into play.
From EP-A- 0.647.064 compensation means are known, which are based on the periodical defocussing of the imaging means. The disadvantage is that at the moment of defocussing the detection arrangement cannot be used for the execution of its proper tasks.
From EP-A- 0.849.941 compensation means are known which utilize the movement of the detection arrangement, resulting in the image on the array moving, too. From the noise that is seemingly added to the image as a result of offset errors, the offset for each detector element can be deduced. Disadvantages of this solution are that it does not work when the detection arrangement is immobile, and that the deduction of the offset requires much calculating effort.
The present detection arrangement has none of these disadvantages and is, according to an aspect of the invention, characterized in that the compensation means comprise a displacement device, positioned between the imaging means and the array and arranged for displacing the image on the array, and filtering means, incorporated in the image processing means, for generating offset compensating values C(i,j) from at least two images with mutually divergent displacements.
A preferred embodiment according to an aspect of the invention is characterized in that the displacement device comprises a rotatably positioned plate that is transparent for infrared radiation. By turning the plate through a previously determined angle, a defined displacement of the image on the array can be effected, after which by comparing the non-displaced image with the displaced image the offset of all detector elements can be established. The displacement of the image may subsequently be cancelled by executing an inverse displacement in the image matrix.
A further preferred embodiment, enabling a number of displacements to be effected in a simple manner, is characterized in that the plate is rotatably positioned around an optical axis, and that at least a front face or a rear face is mounted non-perpendicular to the optical axis.
A further preferred embodiment, enabling virtually any desired displacement to be effected, is characterized in that the plate is rotatably positioned around two axes, the axes being positioned such that they are at least substantially perpendicular to an optical axis of the plate and that they are mutually perpendicular.
According to a further aspect of the invention, an advantageous implementation of this embodiment is characterized in that the axes are positioned at least substantially parallel to the rows and columns of the array. If the image is displaced in one of the two directions, then a simple shift operation within the rows or within the columns of the image matrix suffices for the thus effected displacement to be cancelled. Preferably the displacement is made to cover a distance corresponding to a distance between two detector elements within a row of within a column.
A further preferred embodiment according to an aspect of the invention is characterized in that the compensation means are arranged for periodically displacing, with the aid of the plate, an image on (p,q) detector elements, and for subsequently activating the filtering means. In that case the filtering means are preferably arranged for iteratively generating offset correcting values C(i,j) for detectors D(i,j) according to an equation
C(i,j)new=C(i,j)old+xcex1(S(ixe2x88x92p,jxe2x88x92q)xe2x88x92S(i,j)), with 0 less than xcex1 less than 1.
As the offset for a detector element changes only slowly, a preferred embodiment of the invention is characterized in that 0.02 less than xcex1 less than 0.2. By selecting for xcex1 a small value, the filtering process is prevented from adding temporal noise to the image.
A further preferred embodiment, requiring little calculating capacity and causing practically no reduction of the available image surface, is characterized in that p xcex5{xe2x88x921,0,1} and q xcex5{xe2x88x921,0,1}. In this case, for determining the offset, a detector element is only compared with its nearest neighbours. This is possible because the offset values between adjacent detector elements prove to be virtually uncorrelated.